Pre-treatment of feed to non-stirred surface bioreactor

ABSTRACT

The invention relates to a process for the pre-treatment of feed to a non-stirred surface heap leach bioreactor by applying in sequence first and second pre-treatment solutions to a feed to a non-stirred surface heap leach bioreactor, in which the first solution has an iron content greater than 5 g/l and pH below 2, and the second solution contains a substantially higher microbial population. The invention further relates to methods of adapting a microbial population for use in a non-stirred surface heap leach bioreactor.

FIELD OF THE INVENTION

This invention relates to a process of pre-treatment of feeds tonon-stirred surface bioreactors and a method of adapting a microbialpopulation for use in a non-stirred surface heap leach bioreactor.

BACKGROUND TO THE INVENTION

A method of treating metal bearing solids using a non-stirred surfacebioreactor is described in U.S. Pat. No. 5,766,930, which isincorporated herein by reference. Such a method uses what is hereinaftertermed a “surface bioreactor”.

Some metal bearing solids, usually flotation concentrates, do notrespond favourably or quickly enough to bio-processing and poorbacterial activity, evidenced by low redox potential of the liquidphase, is normally a symptom of such problems.

During the adaptation of micro-organisms to specific concentrates, ithas been observed that in some instances the micro-organisms eitheradapt very slowly, and in rare cases not at all, to the specificconcentrate. This indicates that the leaching microbes are being subjectto some toxic effect from compounds arising from the concentrate. Thisresults in an extremely long lag phase before the microbe population canincrease in number to high enough levels to oxidize the sulphides. Thisextends the leaching time of the process which reduces the economicbenefits of the process.

The operation of a surface bioreactor typically includes the coating ofa flotation concentrate onto a substrate (typically crushed and sizedrock), stacking the coated rock into a heap and inoculating the heapwith an inoculum, typically via the irrigation of recycled heapeffluent. Inoculation by this method is inefficient due to the naturalstickiness of microbes giving rise to low penetration rates of microbesthrough the heap and the low solution application rates used, typicallyaround 20 l/m²/hour.

After the sulphides have been oxidised, the oxidized portion of the heapis taken down and the oxidised concentrate is washed off the rock. Therock is frequently recycled and coated with fresh concentrate. Theoxidised concentrate is then thickened and, for refractory goldoperations, the thickened oxidised concentrate being processed usingcyanidation to recover the gold. Such a process is frequently tested incolumns in the laboratory.

Where the aforementioned problems with apparent toxicity and slowdistribution of microbes through the coated rock are encountered, theredox potential in the effluent solution only increases slowly, delayingthe onset of oxidation and increasing the overall leaching time.Leaching time is an important consideration in surface bioreactor plantsas increased leaching time gives rise to higher pad area requirementsand increased metal inventory within the pad. In a typical refractorygold project this would yield a substantial cost benefit in terms oflower inventory and reduced capital cost.

OBJECT OF THE INVENTION

It is an object of the invention to provide a pre-treatment solution forfeeds to a non-stirred surface bioreactor and a method of pre-treatingfeeds to a non-stirred surface bioreactor which at least partlyovercomes the abovementioned problems.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a process for thepre-treatment of a feed to a non-stirred surface heap leach bioreactorwhich includes the step of applying in sequence first and secondpre-treatment solutions to a feed to a non-stirred surface heap leachbioreactor, in which the first solution has an iron content greater than5 g/l and pH below 2, and the second solution contains a substantiallyhigher microbial population.

There is further provided the microbial population of the secondsolution to be selectively adapted relative to a heap PLS solution.

There is also provided for the process to include the addition ofadditional sulphuric acid concurrently with the iron rich firstsolution.

There is further provided for the second solution to comprise a solutionfrom a dewatering step of oxidised product from the non-stirred surfacebioreactor, and for the iron rich first solution to comprise are-circulating irrigation solution from a non-stirred surface bioreactorprocess.

There is still further for the microbial population of the secondsolution to have been enriched and adapted in an inoculum generatorwhich has been charged with concentrate, sulphur or other material toselectively increase the population of specific microbes in thesolution.

There is also provided for the microbial population of the secondsolution to contain microbes from an inoculum generator of which thetemperature is operated in a predeterminable temperature range toincrease the population of specific microbes in the solution.

There is further provided for the solution with the adapted microbialpopulation content to contain microbes from an inoculum generator whichis fed with concentrate to select microbes most suitable for specifictoxic compounds contained in the concentrate.

There is still further provided the solution with the adapted microbialpopulation content to contain microbes from an inoculum generator whichis fed with a sulphur species to select microbes most suitable forsulphur oxidation.

There is also provided for at least part of the solid phase to beseparated from the liquid phase after pre-treatment, either prior totreatment with another solution or prior to coating and stacking onto anon-stirred surface bioreactor.

The invention further provides for a method of adapting a microbialpopulation for use in a non-stirred surface heap leach bioreactor whichincludes a microbe selection step in which a solution which contains themicrobes is passed through an inoculum generator which is fed withconcentrate to select microbes most adapted for specific toxic compoundscontained in the concentrate in the non-stirred surface heap leachbioreactor, an inoculum generator which is fed with a sulphur species toselect microbes most suitable for sulphur oxidation in the non-stirredsurface heap leach bioreactor, further alternatively an inoculumgenerator which is operated in a specific temperature range toselectively favour a specific microbe most suitable for the temperatureoperating regime used in the bioreactor in the non-stirred surface heapleach bioreactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are shown in the accompanying drawings inwhich:

FIG. 1 shows an embodiment of the invention for processing of a toxicgold bearing arsenopyritic concentrate, containing substantial carbonatemineralization and a tendency to produce elemental sulphur during thebio-oxidation process; and

FIG. 2 shows another embodiment of the invention for processing apyritic gold bearing concentrate with minor carbonate and stibnite thatshows minor toxicity in laboratory amenability tests.

DETAILED DESCRIPTION OF THE INVENTION

Two factors are important regarding the required leaching time for aparticular concentrate in a surface bioreactor.

Firstly, some concentrates appear to give rise to toxicity effects whichleads to an extended lag phase with the bioleaching microbes. Thereasons for this behaviour are not understood entirely, but it may bedue to organic compounds which are present in the concentrate, forexample flotation reagents. Another reason may be the presence of arelatively fast leaching mineral species that releases toxic compoundsinto the solution which inhibits the action of the micro-organisms. Ineither case, removal in part or whole of the organic and/or reactionproducts from the fast leaching species, prior to stacking on the heap,would be beneficial. Toxicity is evidenced by a poor redox potentialpersisting in effluent solutions for several weeks until the bacterialpopulation becomes well established, whereupon the redox rises to >600mV.

Secondly, it is well known that bioleaching microbes excrete exopolymersand are “sticky” in their natural state. Thus inoculation of a surfacebioreactor using irrigation solution results in slow penetration ofbacteria in the heap, also increasing period before leaching beginsefficiently. The natural growth of the leaching microbes that havepenetrated the heap will be further inhibited by any toxic compoundscoming into the solution phase.

Flotation concentrates processed using surface bioreactors are usuallycoated, as thickened slurry from a flotation plant, onto the rock.Alternatively the concentrate may be filtered or dried, re-pulped withwater, and then coated onto the rock. The density of the concentratepulp is an important factor in maintaining adherence of the concentrateto the rock. The concentrate could be re-pulped to the correct densitywith inoculum (for example re-circulating PLS solution), however anytoxic compounds would be stacked along with the concentrate.

In another mode for concentrates containing toxic compounds, improvedbacterial activity in the stacked heaps can be achieved by pre-treatingthe flotation concentrate with an acidic solution of ferric sulphatecontaining substantial quantities of leaching microbes, in one or morepre-treatment reactors.

Large volumes of solution can be rapidly mixed with the incomingconcentrate in a pre-treatment reactor. To apply a similar volume ofsolution via the irrigation system would be much slower. For example, toapply 10 m³ of solution to a tonne of concentrate stacked in a heap withrock would take about 27 days using an irrigation system, assuming anirrigation rate of 20 l/m²/hour and 1.3 t concentrate stacked per m². Incontrast, 10 m³ of solution per hour could be contacted with one tonneof solids every hour in a pre-treatment reactor.

The ability to rapidly mix large volumes of solution with theconcentrate has five important direct and indirect benefits.

Firstly, the quantity of leaching microbes available for leaching isimproved in situ as the coated rock is stacked. If the heap isinoculated using irrigation solution and that solution contains 1×10⁶microbes per ml, some 7.7×10⁶ microbes would be applied per g ofconcentrate in the 27 day period. Whilst the microbial population wouldnormally naturally increase, such increase in population will besubstantially inhibited in the presence of any toxic compounds presentin the solution. Also given the low solution application rates in heaps,such effects are likely to persist with time. Using the pre-treatmentreactor about 1×10⁷ microbes can contacted with a gram of concentrateevery hour. Again though, any toxic compounds will remain in solution.

Secondly, large volumes of ferric solution provide an environment wherefast leaching mineral phases can be leached, especially those producingpotentially toxic products such as As³⁺ from the leaching ofarsenopyrite, albeit partially. Using irrigation solution at, say 15 g/lFe³⁺, at a rate of 20 l/m²/hour onto a heap containing 1.3 t/m² ofconcentrate, applies ferric at a rate of 231 g per hour per tonne ofconcentrate. Consider a concentrate containing 5% of a fast leachingcompound requiring 1:1 ferric addition on a mass basis i.e. the 50 kg offast leaching species per tonne concentrate requires 50 kg of ferric. Itwould take 216 hours to apply the ferric using the irrigation solution.Using a pre-treatment reactor at 10 m³ per tonne concentrate withsimilar ferric concentrations, ferric is applied at a rate of 150,000 gper hour per tonne concentrate, three times the amount required to leachour hypothetical fast leaching species.

A third and indirect benefit is that mixing of a large volume ofsolution with the concentrate requires that the resulting product bedewatered using a thickener and/or filter. Such a process step providesan opportunity to bring the concentrate from the flotation plantconsistently to the correct pulp density, which is a very importantfactor in maintaining adherence of the concentrate to the rock.

Fourthly, any toxic compounds are diluted substantially and, coupledwith the dewatering step above, a large proportion of any soluble toxiccompounds associated with the liquid phase may be removed andimmediately discarded. Additionally the concentration of toxic compoundsin the liquid phase of the coating on the rock is reduced directly priorto stacking.

The fifth benefit is that many flotation concentrates contain acidconsuming carbonate minerals. These carbonates may be removed withsulphuric acid and concentrated sulphuric acid is usually used for this.However the direct addition of sulphuric acid to a carbonate containingconcentrate usually results in severe foaming and expansion of theslurry, which presents practical difficulties. However the addition oflarge solution volumes enables concentrated sulphuric acid to be mixedinto the slurry without concerns of foaming. Also the acid generated inthe PLS by bioleaching pyrite in the concentrate can be put to use.Additionally, because the carbonate is removed upfront, the pH in thePLS drops quickly to that suitable for bioleaching, reducing therequired time on the leach pad. For example to add 100 kg/t of acid to aconcentrate via irrigation solution containing 5 g/l sulphuric acid at arate of 20 l/m2/hour would take 1000 hours, but a similar amount of acidcan be applied using PLS solution in a matter of hours, especially witha top-up of fresh acid.

The acidic solution of ferric sulphate containing substantial quantitiesof leaching microbes may be tailored for specific applications byjudicious use of available plant solutions. The inventors have observedthat the re-circulating irrigation solution usually has quite a highpopulation of microbes typically around 1×10⁶ per ml and is enriched iniron, typically 5-40 g/l Fe with a pH of about 1.5. However the overflowfrom the reclaim thickener (that dewaters the oxidised concentrate)typically contains about an order-of-magnitude higher content ofmicrobes at around 1×10⁷ per ml, but is normally low in iron content(the heap having been rinsed with water prior to being taken down)typically <2 g/l Fe and higher pH at around pH3. Additionally, microbesfrom the reclaim thickener overflow have been washed off the heap massand are likely more adapted to conditions within the heap, whereas thosein the irrigation solution may not. Thus using various mixtures of thereclaim thickener overflow and re-circulating irrigation solution in thepre-treatment steps, the type and quantity of leaching microbes and ironcontent of the solution phase in the pre-treatment steps may be tunedfor a specific concentrate. Additional sulphuric acid may be added asappropriate to dissolve carbonate minerals.

Additionally some or all of the irrigation solution and/or the reclaimthickener overflow solution may have their microbe content enriched by amicrobe selection step. By passing the solutions through an inoculumgenerator, fed with small amounts of concentrate to select microbes mostadapted to any toxic compounds contained in the concentrate. Theinoculum generator may be fed using other materials (for examplesulphur) to select the population of a particular microbial species thathas the desired genetic trait for example sulphur metabolism.Alternatively it may be operated in a specific temperature range, toincrease the population of a desired microbe (for example extremethermophiles, by operating at >60 Deg C.). It may also be possible touse a combination of feeding small amounts of concentrate to theinoculum generator and operating it in specific temperature range toselect the most suitable microbe.

Whilst the cost of pre-treatment reactors, thickeners and inoculumgenerators will not be insignificant, the savings due to a reduction inleaching period will typically be much higher.

FIG. 1 shows a surface bioreactor flowsheet for the treatment of thegold bearing arsenopyrite concentrate showing severe toxicity inlaboratory tests, with major carbonate content. The oxidised concentratehas high levels of elemental sulphur.

A bleed stream (120 a) from the irrigation solution pond (120) is mixedwith the incoming flotation concentrate (1) in a first pre-treatmentreactor (30), to which 100 kg/t of sulphuric acid (31) is added. Theleached solids (30 b) are fed to a pre-treatment thickener (40). Thepre-treatment thickener overflow (40 a) goes to neutralisation (60) anddisposal. The pre-treatment thickener underflow (40 b) is fed to asecond pre-treatment reactor (50), where a solution (100 d) from aninoculum generator (130) fed with sulphur (130 a) and a bleed (100 b) ofthe oxidized concentrate thickener overflow (100 a) is added. The secondpre-treatment reactor product (50 b) goes to a coating device (70) alongwith recycled support rock (200). The coated support rock (70 a) is fedto the surface bioreactor heap (80).

The surface bioreactor heap (80) is continuously irrigated with solution(120 a) derived from the irrigation solution pond (120). The heapeffluent solution (80 a) flows back to the irrigation solution pond(120). Low pressure air (117 a) is blown through the heap surfacebioreactor (80).

Once oxidation is completed the oxidised portion (80 b) of the surfacebioreactor heap (80) is removed and fed into an oxidized concentratescreen (90). The washed support rock (200) is fed to the coating device(70). The oxidized fines (90 b) are fed to an oxidized concentratethickener (100) from which the thickener underflow (100 d) is furtherprocessed using cyanidation to recover the gold. The oxidizedconcentrate thickener overflow (100 a) is split into a portion (100 c)going to the solution pond (120) and a bleed portion (100 b) going to aninoculum generator (130) fed with elemental sulphur (130 a).

FIG. 2 shows a surface bioreactor flowsheet for the treatment of thegold bearing pyrite concentrate, with minor carbonate and stibnitecontent.

A bleed stream (25 a) from the irrigation solution pond (25) is mixedwith the incoming flotation concentrate (21) in a first pre-treatmentreactor (22), to which some sulphuric acid (23) is added. The leachedsolids (26) are fed to a second pre-treatment reactor (26 a), where ableed (27) of the oxidized concentrate thickener overflow (28 a) isadded. The second pre-treatment reactor product (29) is thickened in apre-treatment thickener (29 a). The pre-treatment thickener overflow (29b) goes to neutralisation (11) and disposal. The pre-treatment thickenerunderflow (12) is fed to a coating device (13) along with recycledsupport rock (20). The coated support rock (14) is fed to the surfacebioreactor heap (15).

The surface bioreactor heap (15) is continuously irrigated with solution(17) derived from the irrigation solution pond (25). The heap effluentsolution (16) flows back to the irrigation solution pond (25). Lowpressure air (17 a) is blown through the heap surface bioreactor (15).

Once oxidation is completed the oxidised portion (18) of the surfacebioreactor heap (15) is removed and fed into an oxidized concentratescreen (19). The washed support rock (20) is fed to the coating device(13). The oxidized fines (21) are fed to an oxidized concentratethickener (28) from which the thickener underflow (28 b) is furtherprocessed using cyanidation to recover the gold. The oxidizedconcentrate thickener overflow (28 a) is split into a portion (22 a)going to the solution pond (25) and a bleed portion (27) going to thesecond pre-treatment reactor (26 a).

It will be appreciated that the embodiments described above has beenincluded by way of example only, and is not intended to limit the scopeof the invention. It is possible to alter certain aspects of theembodiment within the scope of the invention.

It is, for example, possible that the invention can be used for theprocessing of base metal concentrates, as well as in all cases where oremay be used as the substrate. It is also possible to use the inventionin stirred tank processing of gold and base metal concentrates.

It is also possible to include additional a thickener in the processbefore the coating step, which may be required to increase the solidscontent of the material to be coated on the feed to the bioreactor.Referring to FIG. 1, such a thickener could be located betweenPre-treatment Reactor 2 (50) and the Coating step (70), in other wordsin stream 50 b.

1. A process for the pre-treatment of a feed to a non-stirred surfaceheap leach bioreactor which includes the step of applying in sequencefirst and second pretreatment solutions to a feed to a non-stirredsurface heap leach bioreactor, in which the first solution has an ironcontent greater than 5 g/l and pH below 2, and the second solutioncontains a substantially higher microbial population.
 2. The process asclaimed in claim 1 in which the microbial population of the secondsolution is selectively adapted relative to a heap PLS solution.
 3. Theprocess as claimed in claim 1 in which includes adding additionalsulphuric acid concurrently with the iron rich first solution.
 4. Theprocess as claimed in claim 1 in which the second solution comprises asolution from a dewatering step of oxidised product from the non-stirredsurface bioreactor.
 5. The process as claimed in claim 1 in which theiron rich first solution comprises a re-circulating irrigation solutionfrom a non-stirred surface bioreactor process.
 6. The process as claimedin claim 2 in which the microbial population of the second solution hasbeen enriched and adapted in an inoculum generator which has beencharged with concentrate, sulphur or other material to selectivelyincrease the population of specific microbes in the solution.
 7. Theprocess as claimed in claim 2 in which microbial population of thesecond solution contains microbes from an inoculum generator of whichthe temperature is operated in a predeterminable temperature range toincrease the population of specific microbes in the solution.
 8. Theprocess as claimed of claim 2 in which the solution with the adaptedmicrobial population content contains microbes from an inoculumgenerator which is fed with concentrate to select microbes most suitablefor specific toxic compounds contained in the concentrate.
 9. Theprocess as claimed in claim 2 in which the solution with the adaptedmicrobial population content contains microbes from an inoculumgenerator which is fed with a sulphur species to select microbes mostsuitable for sulphur oxidation.
 10. The process as claimed in claim 1 inwhich at least part of the solid phase is separated from the liquidphase after pre-treatment, either prior to treatment with anothersolution or prior to coating and stacking onto a non-stirred surfacebioreactor.
 11. A method of adapting a microbial population for use in anon-stirred surface heap leach bioreactor which includes a microbeselection step in which a solution which contains the microbes is passedthrough an inoculum generator which is fed with concentrate to selectmicrobes most adapted for specific toxic compounds contained in theconcentrate in the non-stirred surface heap leach bioreactor.
 12. Amethod of adapting a microbial population for use in a non-stirredsurface heap leach bioreactor which includes a microbe selection step inwhich a solution which contains the microbes is passed through aninoculum generator which is fed with a sulphur species to selectmicrobes most suitable for sulphur oxidation in the nonstirred surfaceheap leach bioreactor.
 13. A method of adapting a microbial populationfor use in a non-stirred surface heap leach bioreactor which includes amicrobe selection step in which an inoculums generator is operated in aspecific temperature range to selectively favour a specific microbe mostsuitable for the temperature operating regime used in the bioreactor inthe non-stirred surface heap leach bioreactor.